1. Field of the Invention
The present invention relates to an adhesive tape for joining webs. More particularly, the present invention is concerned with an adhesive tape for joining photographic supports using high frequency heating.
2. Description of the Prior Art
The term "high frequency" as used herein designates alternating waves of frequencies of several MHz or more.
In the continuous production of a photographic light-sensitive member, long, flexible photographic supports (hereinafter referred to as "webs") are joined to each other and the resulting endless support is passed continuously through a series of processing stations, the surface of the running support being coated with a photographic light-sensitive material, and then dried and wound up.
Various methods are known for joining such webs, and of these methods, joining by the use of an adhesive tape is widely used.
However, the glue joints of webs joined with an adhesive tape are thick as compared with the other areas, and therefore, if the glue joints are left as they are, the following problems occur:
1. Where the clearance between a coating device and the web, as in the case of bead coating, is extremely small, it is impossible for the joint to pass the coating unit; PA1 2. Even if the joint can pass the coating unit, uncoated areas and extremely thickly coated areas are formed on the surface of the web at the rear side of the joint, or stripes are formed on the coating film for a considerable length behind the joint (which are due to air bubbles); and PA1 3. Even if coating is successfully carried out, the joints strongly press against adjacent webs when the web is wound up, and thus pressure fog is liable to result. PA1 1. By knowing the position of a joint, it is possible to enlarge the clearance between the coating apparatus and the web when the joint passes through the coating unit. As a result, the joint passes smoothly through the coating unit; PA1 2. Even if the position of the joint is known, it is impossible to avoid uneven coating, etc., behind the joint. However, it is possible to cut off all unsuitable areas including the joints prior to winding by detecting the position of the joint; and PA1 3. Since the joint can be cut off prior to winding by knowing the position of the joint, there is no danger of pressure fog.
To overcome these problems, it is essential to detect the position of a joint, at least during production, because:
Accordingly, an adhesive tape comprising an aluminum film (hereinafter, "aluminum tape") which can be easily detected, has heretofore been used as a tape for joining webs.
FIG. 1 shows a sectional view of the aluminum tape hitherto used, wherein 1 is an undercoating layer, 2 is a support, 3 is an aluminum layer, 4 is an adhesive layer, and 5 is a release paper. The aluminum layer 3 is produced by depositing an extremely thin aluminum film on support 2, and the release paper 5 is peeled off the adhesive layer 4 at the time of use.
The conductivity of such an aluminum tape itself is high, and eddy currents easily flow therein. Thus, for example, a joint can be detected by a change in the magnetic flux resulting when two webs joined by the aluminum tape pass a magnetic head. Furthermore, this detection method does not deteriorate the quality of a photographic film. For these reasons, aluminum tape has commonly been used in the field of producing photographic supports.
Recently, however, requirements for rapidly drying web coatings on photographic light-sensitive members to increase production efficiency have increased.
As one such rapid drying method, drying by high frequency heating as described in U.S. Pat. No. 3,491,457 and Japanese Patent Laid Open No. 81953/1974 (hereinafter referred to as "high frequency drying") has been proposed.
This high frequency drying is usually carried out by simultaneously irradiating with high frequency irradiation, 915 MHz or 2450 MHz, and blowing a high velocity gas stream on the web. The apparatus for this processing is provided with a waveguide and a gas nozzle, and the web is subjected to high frequency irradiation and to the high velocity gas stream when it passes between the slits of the waveguide. Since such high frequency drying dries the web uniformly and rapidly while maintaining the surface of the web at a suitable temperature, the use of such high frequency drying for photographic light-sensitive materials is increasing.
It is known, however, when a metallic material of high conductivity is placed in a high frequency electric field, a spark or local current occurs between the drying apparatus and the metallic material, resulting in development of localized increased temperature areas in the metallic material and a burning of the metallic material. Since aluminum tape has a thin aluminum film of extremely high conductivity, the aluminum tape sparks with the drying apparatus in a high frequency electric field and is burned. As a result, not only is the strength of the joint decreased, but serious problems such as fogging of the photographic light-sensitive member due to sparking result. Thus, it has been difficult to use high frequency drying to dry photographic materials comprising webs joined by aluminum tape.